Positive displacement pump for dispensing a metered quantity of liquid

ABSTRACT

A liquid dispensing pump is disclosed which is driven by compressed gas and which withdraws liquid from a source of supply under partial vacuum. The pump includes means for positively purging the driving gas at the end of the dispensing stroke, so that the pump will quickly be ready for another dispensing action if desired. The pump includes a provision for volume adjustment, and the volume adjustment mechanism includes a special anti-rebound feature.

United States Patent 1 i1 1 Slagle [451 July 31,1973

[ POSITIVE DISPLACEMENT PUMP FOR DISPENSING A METERED QUANTITY OF LIQUIDInventor:

William E. Slagle, Barrington, Ill.

Lincoln-Hall Research Company, Lincolnwood, Ill.

Mar. 16, 1972 Assignee:

Filed:

Appl. No.:

U.S. Cl 417/395, 91/234, 91/402 Int. Cl. F04b 43/10, F04b 45/06 Field ofSearch 417/402, 392, 401,

Primary Examiner-William L. Freeh Assistant Examiner-Richard SherAttorney-Vernon D. Beehler. Gene W. Arant et a1.

[57] ABSTRACT A liquid dispensing pump is disclosed which is driven bycompressed gas and which withdraws liquid from a source of supply underpartial vacuum.

The pump includes means for positively purging the driving gas at theend of the dispensing stroke, so that the pump will quickly be ready foranother dispensing action if desired.

The pump includes a provision for volume adjustment, and the volumeadjustment mechanism includes a special anti-rebound feature.

12 Claims, 7 Drawing Figures 50 fi /60 40 5 55 62 /09 6 I //7 A25 #6 fin e7: 205 /2/ 224 3 5 a //5 /2 i //Z I 5 e 1 .5 20, I: /9/ [i //9 E63r- T 7 T: %5 l L. T

2 n 1, /70 u ii /92 l /4, 495 /4 f /44 /45 r /4/ /4d 59 A48 6 l" 1 6 M3M2 Patented July 31, 1973 3,749,527

4 Sheets-Sheet 1 70 CASH 1956/572 Patented July 31, 1973 3,749,527

4 Shouts-Sheet i5 Patented July 31, 1973 3,749,527

4 Sheets-Sheet 4 POSITIVE DISPLACEMENT PUMP FOR DISPENSING A METEREDQUANTITY OF LIQUID BACKGROUND OF THE INVENTION The present invention isan improvement on Beverage Dispensing Apparatus which is disclosed inthe co-pending application of Wilbert .l. Jaeger, Ser. No. 38,800, filedMay 19, 1970, which is assigned to the same assignee as the presentapplication.

One aspect of the present invention is the provision of novel means forpurging the driving gas from the gas expansion chamber as a directresult of the forward movement of the piston.

Another feature of the present invention is the provision of twoseparate and independent means for purging the driving gas, whichoperate in parallel and hence supplement each other.

A third aspect of the invention is the incorporation of an anti-reboundfeature into the volume adjustment means.

It is therefore the object and purpose of the present invention toprovide a new and improved and more reliable positive displacement pumpfor dispensing a precisely metered quantity of a liquid.

DRAWING SUMMARY FIG. 1 is a perspective view of a liquid dispensing unitin accordance with the invention;

FIG. 2 is a cross-sectional view of the pump and its associated controlhousing, shown in one position of the working parts;

FIG. 3 is a view like FIG. 2 but showing a different position of theworking parts;

FIG. 4 is an enlarged fragmentary cross-sectional view ofa two-partvalve mechanism utilized in the control housing;

FIG. 5 is a transverse cross-sectional view taken on the line 5-5 ofFIG. 4;

FIG. 6 is a transverse cross-sectional view taken in the line 6-6 ofFIG. 2; and

FIG. 7 is a cross-sectional view of the pump showing an alternateposition of the volume adjustment mechamsm.

PREFERRED EMBODIMENT Reference is now made to FIGS. 1 to 7, inclusivewhich illustrates a complete liquor dispensing unit in accordance withthe present invention. FIG. 1 shows a dispensing nozzle assembly N, aliquor bottle 20, a liquor dispensing line 30 which is coupled to thenozzle assembly, a pressure relief line 40 coupled to the nozzleassembly for controlling the dispensing of liquor, and a pump P for theliquor.

The pump P shown generally in FIG. 1 (its internal construction beingshown in detail in FIGS. 2, 3 and 4) includes a main housing 100 withinwhich the dispensing operation is performed, and an auxiliary or controlhousing. The control housing includes an upper block 150, a middle block160, and a lower block 170. A tube section 31 extending from liquorbottle is coupled to the upper end of dispensing housing 100 through avalve unit 102. A tube section 32 supplies liquor at atmosphericpressure to the interior of bottle '20. Dispensing tube leading to thenozzle N is also coupled to the upper end of dispensing housing 100,through a valve unit 104. Adjusting handle 145, which is used foradjusting the precise amount of liquor to be dispensed in each shot,protrudes from the bottom of housing 100.

The pressure relief line 40 from the nozzle assembly N is coupled intothe center section 160 of the control housing. Gas pressure line 205receives pressurized gas from a tank C for energizing the dispensingoperations, and is coupled into the lower section 170 of the controlhousing.

FIG. 1 also illustrates an indicator system which may be used asoptional equipment. A pulse transducer 195 is coupled to the lowersection 170 of the control housing. The pulse transducer is energizedfrom an electrical cable 196 and supplies counting pulses via electricalcable 198 to a cash register. The operation of lower section 170 of thecontrol housing (FIGS. 2, 3 and 4) is such that each time a shot ofliquor is dispensed a pulse of gas pressure is applied to the pulsetransducer 195.

The operation of the displacement pump P is such as to draw a vacuum onthe line 31. The manner in which liquor is drawn by vacuum from bottle20 is described in detail in the copending application of William E.Slagle, Ser. No. l93,l37, filed Oct. 27, 1971 and assigned to the sameassignee as the present application.

The dispensing housing contains a chamber having forward end portion andrearward end portion 112, and a piston 130 which reciprocates betweenthe end portions of the chamber. In the retracted position of piston 130as shown in FIG. 2 the chamber portion 110 holds a precisely measuredquantity of liquor. As the piston 130 travels. forward the liquor isdispensed through valve fitting 104 into output line 30. FIG. 3 showsthe piston as it is moving forward and prior to striking the end wall113 of the chamber.

More specifically, the dispensing housing 100 on its upper end has apassageway 101 which opens through the end wall 113 of the chamber 110,112. A standard check valve unit 102 is screwed into the outer end ofpassageway 101, and receives the end of tube 31. Valve fitting 102contains a check valve which permits liquor to flow from the tube 31into the chamber 1 10, but will not permit flow in the oppositedirection.

In similar fashion a passageway 103 communicates from the upper end ofhousing 100 through the wall 1 13 of the chamber. A valve fitting 104 isscrewed into the outer end of passageway 103, and receives the outputtubing line 30. Valve fitting 104 contains a check valve which permitsliquor to flow from the chamber out into 30, but not in the oppositedirection.

A separate piston housing provides the chamber portion 112 within whichpiston may be partially retracted. Piston housing 1 15 is inserted intothe otherwise open lower end of dispensing housing 100 to confine thepiston 130 in the chamber thereby provided. The piston 130 is ofgenerally cylindrical configuration and is covered by a flexiblebellofram 132 which covers the lower end of the piston as well as itscylindrical sidewall and extends peripherally outwardly from the piston.The peripheral extremity of bellofram 132 is retained between annularshoulder 109 of housing 100 and the upper circumferential end face ofthe piston housing 1 15.

A metal end cap 131 fits over the lower end of piston 130 to retainbellofram 132 in place. A piston stem or rod 137 has its upper endthreaded at 135 and screwed into a threaded opening in the piston 130. Anut 136 carried on the threaded portion of stem 137 is tightened againstthe metal cap 131 for retaining the piston and pistion rod in assembledrelationship, and at the same time firmly pressing the end cap 131against the bellofram. When the piston 130 traveis forwardly thebellofram 132 is progressively peeled back from the circumference of thepiston, and hence serves to maintain a tight pressure seal between thechamber portion 110 and the chamber portion 112 (see FIGS. 3 and 4).

Pressurized gas flows into compartment 112 through a horizontalpassageway 106, annular space 111 surrounding the piston housing, and apassageway 116. The pressurized gas is supplied to passageway 106through a communicating passageway 193 of the lower control housingblock 170. Whenever piston 130 is to be driven in an upward strokepressurized gas from the line 205 is permitted to pass through thecontrol mechanism into passageway 193 and hence into compartment 112.

The dispensing housing 100 also has a sensing passageway 105 formedtherein, which communicates from the upper end of chamber portion 1 tothe control housing 150. The purpose of passageway 105 is to sense theliquid pressure in the chamber portion 110, the response to this liquidpressure being utilized in the control system in a manner which willsubsequently be described. The net flow of liquor back and forth in thesensing passageway 105 is of miniscule proportions, however, and ittherefore does not affect the precision metering action achieved by thechamber 110.

The operation of pump P in dispensing a precisely measured amount ofliquor has been described in the copending application of Wilbert J.Jaeger, Ser. No. 38,800, filed May 19, l970,and which is assigned to thesame assignee as the present application. According to the presentinvention, however, a somewhat different control action is utilized.Furthermore, according to the present invention a novel method isprovided for purging the pressurized gas when the drive stroke iscompleted.

CONTROL OF PUMP P The method of controlling the operation of dispensingpump P is, in general, as follows:

The supply of compressed gas from the supply line 205 to the chamber 112of the pump is controlled by a pilot-operated valve. This valveincluding movable members 174 and 180 is shown in its closed position inFIG. 2, while in FIG. 3 it is shown in its open position. Thepilot-operated valve is in turn controlled by a pressure-sensitivediaphragm 153. FIG. 2 shows the position of diaphragm 153 when thesystem has been pressurized and is ready to operate, while FIG. 3 showsthe alternate position which the diaphragm assumes after pressure hasbeen relieved through pressure relief line 40 by actuation of a buttonon nonle N.

The control housing 150, 160, 170 and mechanisms contained therein willnow be described in detail.

In the upper control housing section 150 a sensing passage 151communicates between sensing passage 105 of housing 100, and-a chamber152 which opens onto the upper side of a diaphragm 153.

Control housing section 160 has a raised surface portion 162 which isspaced only a short distance away from the under surface of diaphragm153, when diaphragm 153 is in its raised position as shown in FIG. 2.Both the pressure line 205 and pressure reiief line 40 communicate withthe under side of diaphragm 153.

Pressure line 205 communicates through an opening formed in the raisedsurface portion 162. When the button is pushed and the pressure in line40 is relieved. there is a drop in pressure on the under side ofdiaphragm 153, permitting that diaphragm to close on the raised portion162.

The separate blocks 150 and 160 are conveniently arranged so thatdiaphragm 153 is supported between the under side of biock 150 and theupper side of block 160.

A second diaphragm 167 is supported between the under side of block 160and the upper side of block 170. Pressure from line 205 is applied tothe under side of diaphragm 167, but is bled off when line 40 is open.

The under side of diaphragm 167 is coupled to the upper member 174 of atwopart valve mechanism 174, 180. When line 40 is opened (by pushing aliquor dispensing button on the nozzle assembly N), after the movementof diaphragm 153 takes place as previously described the pressure on theunder side of diaphragm 167 rises and overcomes the counterbaiancingeffect of associated spring mechanisms. Diaphragm 167 and member 174then move upward, admitting gas from line 205 into drive passageway 193.Piston 130 is then driven upward in a dispensing stroke.

Upon completion of the dispensing stroke of piston 130 there is apressure decay in liquor chamber 110, and a consequent pressure drop insensing passageway which is communicated to diaphragm 153. The resultingmovement of diaphragm 153 causes diaphragm 167 to move, driving valvemember 174 downward and shutting off the supply of pressure gas tocompartment 112.

The downward movement of valve member 174 supplies a pressure pulsethrough passageway 192 to transducer device 195, which then registersthe dispensing of one shot of iiquor.

The downward movement of valve member 174, returning it to its positionshown in FIG. 2, also provides a pathway for purging gas fromcompartment 1 12. The gas flows through passageways 116, 111, 106, 193,around the valve member 180, and through passageway 191 to atmosphere.The decay of pressure in compartment 112 permits the piston to bereturned to its starting position by action of its return spring.

Moreover, according to the present invention a positive mechanicalpurging action is provided, which acts in parallel and in addition tothe purging action just described. The mechanical purging action will bedescribed in conjunction with a more detailed description of the pistonmechanism.

PISTON HOUSING AND MECHANICAL PURGE Having now established thebackground of the invention, in respect to which its operation isessentiaily identical to what is described in the previously referencedJaeger application, the novel features of the invention will now bedescribed in detail.

The piston housing is generally cylinderical in configuration, butsomewhat above its center it has a transverse horizontal wall 121 formedtherein. A forward or upper recess 117 is thereby formed, which is ofcylindrical configuration and is adapted to partially receive the pistonwhen the same is withdrawn downwardly from the liquor chamber. In oneside wall of the forward recess 117 a passageway 116 is formed foradmitting compressed gas for driving the piston forseal 123 as thepiston 130 is driven forward and retracted. A central hub 125 is formedon the under side of transverse wall 121 and depends downwardlytherefrom. The vertical height of the central hub 125 is abouttwo-thirds the vertical height of the piston housing 115, and its lowerend is coterminous with the lower end of the piston housing. Central hub125 has a cylindrical center hole 126 formed therein, which extendsdownwardly from the central recess 122, and the piston rod 137 (althoughother portions of its length are designated differently) slides withinthe full length of the hole 126. Near the upper end of hub 125, and onlya short distance below the transverse wall 121 and O-ring seal 123, ahorizontal passageway 127 is formed which communicates between theexternal surface of the hub 125 and its center hole 126.

Beneath the transverse wall 121 the piston housing 115 has a rearward orlower recess 120 formed therein, which is also generally cylindrical andwhich has a threaded wall 119. The threaded wall 119 receives the endcap or adjustment handle 145, for purpose of volume adjustment.

The piston rod 137 has one portion of its length, approximatelyintermediate its two ends, which has a reduced diameter and isdesignated by numeral 138. The distance of reduced diameter portion 138of the piston rod from the piston 130 is such that, in the extremeforward position of the piston as shown in FIG. 3, the reduced diameterportion of the piston rod extends from a point just below the passageway127 in hub 125 to a point just above the O-ring seal 123. In thisposition of the piston and piston rod, as would be evident from FIG. 3,compressed gas contained within expansion chamber 112 is permitted tofreely pass through the central recess 122 and through the O-ring seal123 and a short section of the hub center hole 126 and thencehorizontally through the passageway 127 into the lower recess 120 of thepiston housing. Thus, a positive mechanical purge of the gas expansioncompartment 112 is provided.

The purged gas passes downward from chamber 120 into the interior ofadjusting handle or cap 145, where it escapes into atmosphere through aslot or opening 148 which is provided specifically for that purpose. Theopening 148 is located near the lower end of the cap 145 so that itoperates for its intended purpose even when the pump is adjusted fordispensing smaller quantitles of liquor, as shown in drawing FIG. 7.

A lock nut 146 is utilized to fasten the cap 145 in a selected positionof adjustment relative to piston housing 115. The return spring for thepiston is designated 141, and has its upper end in engagement with thelower surface of the transverse wall 121 and immediately surrounding thecentral hub 125. Spring 141 extends below the hub 125 and its lower endis held by a spring retainer 142 which is in turn attached to the lowerextremity 140 of the piston rod by means of a lock washer 143.

Below the reduced diameter portion 138 the piston rod is designated 139,and the portion 139 has the same size or outer diameter as the forwardor upper portion 137. A positive support for the piston and its pistonrod is provided by the central hub 125, since the length of reduceddiameter portion 138 of the piston rod is quite small compared to thelength of the hub.

Although it would not be necessary, the lower extremity of the pistonrod is also formed with a reduced diameter, being designated as 140.Spring retainer 142, although held onto the piston rod by lock washer143, may slide a short distance lengthwise on the end portion 140 of thepiston rod. The adjusting handle or end cap has a central recess 147formed in its lower end, immediately beneath the piston rod. When thepiston is withdrawn from the liquor chamber it is done so by the actionof spring 141 which forces the spring retainer 142 downward. Springretainer 142 bottoms on the end wall of the end cap 145. Piston 130 andthe piston rod may over travel a small distance, however, in which casethe lower extremity 140 of the piston rod dips momentarily into recess147 of the end cap. This action avoids a hard rebound of the piston rodand also avoids an undesired secondary dispensing action. This featureof the pump is particularly advantageous where, as shown in FIG. 7, forexample, the adjustment cap has been set inward to provide fordispensing of a relatively small quantity of liquid. Where the chamberis set for its maximum volume, as shown in FIG. 2, the rearward travelof the piston is stopped by the transverse wall 121 and there is norebound problem of observable magnitude.

Another feature of the apparatus is shown in FIG. 6 where it will beseen that a number of longitudinally extending ribs 144 are providedwithin the end cap 145. Ribs 144 fit relatively closely around thespring 141 and prevent it from getting out of alignment. At the sametime the flow of purged gas toward the exit port 148 is not impeded.

A significant feature of the present invention is that driving gas ispurged from the gas expansion chamber 112 by two entirely separate andindependent means. One means is dependent upon the piston 130 strikingthe forward wall 113 of the liquor chamber 110, with resulting pressurechange which is sensed in the passageway 105. Movement of the diaphragms153 and 167 then results, permitting gas to flow through a relativelylong passageway including the passageway 191 to atmosphere. The otherand separate purging action, however, is initiated even before thepistion strikes the end wall 113. As soon as the reduced diameterportion 138 of the piston rod couples passageway 127 to the gasexpansion chamber 112, the purging action commences. The forwardvelocity of the piston and its associated piston rod is sufficient tocarry it through the samll remaining distance that it needs to travel.

Although the volume of the metering chamber is adjustable, it will benoted that such adjustments do not affect the mechanical purging throughthe passageway 127. The reason is that this purging action occurs as thepiston nears the forward extreme of its travel, while volume adjustmentis accomplished by changing the rearward limit of piston movement.

It will also be noted that the considerable length of pistion rodrearward section 139 is sufficient to provide a rigid support for thepiston 130 even at the time when no support is being achieved at theO-ring seal 123. it will further be noted that the relatively shortpathway through the O-ring seal and passageway 127 assures a positivepurging action for the compartment 112.

As will be understood by those skilled in the art, what has beendescribed are preferred embodiments in which modifications and changesmay be made without departing from the spirit and scope of theaccompanying claims.

What is claimed in support of Letters Patent is:

1. A positive displacement pump for selectively dispensing a liquid,comprising:

a cylinder having a reciprocating piston, a piston rod extendingrearwardly from said piston, and sliding bearing means supporting saidpiston rod;

a concentric cylinder resilient seal coupled between said piston and thecylinder wall so as to divide said cylinder into a forward chamber forcontaining a liquid to be dispensed, and a rearward chamber;

means for allowing the liquid to exit from said forward chamber throughan outlet port during the forward travel of said piston, and to entersaid forward chamber through an inlet port during the return travel ofsaid piston;

inlet vent means for selectively admitting pressurized gas into saidrearward chamber for driving said piston forward;

control means operable when said piston reaches the forward limit of itstravel for closing said inlet vent means; 7

said sliding bearing means normally providing a seal with said pistonrod, and said bearing means and piston rod cooperatively providing anoutlet vent for exhausting driving gas from said rearward chamber whensaid piston closely approaches the forward limit of its travel;

and drive means operable for returning said piston to its rearwardposition when the driving gas has been exhausted.

2. A pump as claimed in claim 1 wherein said control means is operablein response to a drop in liquid pressure at the forward end of saidforward chamber which results from exhausting said driving gas from saidrearward chamber through said outlet vent and releasing the liquid fromsaid forward chamber through said outlet part.

3. A pump as in claim 1 wherein said sliding bearing means consists ofan elongated cylindrical member extending rearward from the back side ofsaid rear chamber and having a concentric center bore matching, in closefitting relationship, the piston rod outer diameter;

said elongated member having an exhaust port extending radially fromsaid center bore through said elongated member wall;

said seal consisting of an O-ring seal acting between said elongatedcylindrical member and said piston rod, and in sliding contact with saidpiston rod;

said piston rod having a section of reduced diameter;

and

said outlet vent being provided through the cooperative action of saidpiston rod, bearing means, and seal, said rod reduced sectionsimultaneously engaging said seal and said exhaust port as the pistonclosely approaches the forward limit of its travel.

4. A pump as in claim 3 wherein:

said O-ring seal and said exhaust port are located at the piston rodentrance to the rear chamber thereby providing an exhaust path ofminimum restriction.

S. A positive displacement pump for selectively dispensing a liquid,comprising:

a cylinder having a reciprocating piston, a piston rod extendingrearwardly from said piston, and sliding bearing means supporting saidpiston rod;

a concentric cylinder resilient seal coupled between said piston and thecylinder wall which separates said cylinder into a forward chamber forcontaining a liguid to be dispensed, and a rear chamber;

means for allowing said liquid to exit from said forward chamber throughan outlet part during the forward travel of said piston and enter saidforward chamber through an inlet part during the return travel of saidpiston;

a source of pressurized gas for driving said piston forward;

normally closed inlet vent means for admitting the pressurized gas fromsaid source into said rear chamber; I

nonnally open outlet vent means for exhausting the pressurized gas fromsaid rear chamber;

manually actuable control means for selectively opening said inlet ventmeans and simultaneously closing said outlet vent means to drive thepiston forward;

separate control means operable when said piston reaches the forwardlimit of its travel for simultaneously closing said inlet vent means andopening said outlet vent means;

said sliding bearing means normally providing a seal with said pistonrod, and said bearing means and piston rod cooperatively providing anoutlet vent for exhausting driving gas from said rear chamber only whensaid piston closely approaches the forward limit of its travel;

and drive means operable for returning said piston to its rearwardposition when the driving gas has been exhausted.

6. A pump as in claim 5 wherein said separate control means is operablein response to a drop in liquid pressure at the forward end of saidforward chamber, said drop in liquid pressure resulting from thecombined effects of the exhausting of said driving gas from said rearchamber through said outlet vent and the release of the liquid from saidforward chamber through said outlet part. 1

7. A pump as in claim 5 with a highly restricted bleeder hole consistingof a small diameter hole passing through the wall of said rear chamber,for slowly venting the driving gas from said rear chamber and therebyallowing said piston to return to its rearward limit.

8. A pump as in claim I with an adjustable sleeve for limiting thepiston return thereby providing an adjust able control of the pistondisplacement without affecting the cooperative action of said pistonrod, bearing means, and seal.

9. A pump as in claim 5 with an adjustable sleeve for limiting thepiston return thereby providing an adjustable control of the pistondisplacement without affecting the cooperative action of said pistonrod, bearing means, and seal.

10. A pump as in claim wherein the said drive said piston rod slidinglypasses; means operable for returning said piston consists of a and aretainer member secured to the end of said pisspring driven pistonreturn. ton rod for preventing said piston rod from being 11. A pump asin claim 5 wherein said drive means withdrawn f Said spring retainer;Operable for 'f' l Said Piston isaspring driven P 5 the action beingsuch that when the piston is rereturn :ompnsmgZ turned, the piston'rodis free to overtravel beyond an adjustable sleeve for limiting thereturn travel of said piston and thereby providing displacement control,composed of a cylindrical housing having a counter-bored interior endwall; a cylindrical spring retainer disposed within the end of saidhousing and adapted to engage the raised the normal fully compressedposition of the spring, temporarily extending into the recessed centralportion of said housing end wall, thereby avoiding an undesired reboundof the piston rod from the housing end wall.

circumferential portion of said housing end wall; A P as i claim whereinf j a Spring disposed i hi i housing and having one able sleeve hasradial flbS pro ecting from its inside surend received b said s ri t i 5face thereby providing guidance and stability for the a piston rodconcentric with said spring, said spring said spring and piston rod.

retainer having a central opening through which

1. A positive displacement pump for selectively dispensing a liquid,comprising: a cylinder having a reciprocating piston, a piston rodextending rearwardly from said piston, and sliding bearing meanssupporting said piston rod; a concentric cylinder resilient seal coupledbetween said piston and the cylinder wall so as to divide said cylinderinto a forward chamber for containing a liquid to be dispensed, and arearward chamber; means for allowing the liquid to exit from saidforward chamber through an outlet port during the forward travel of saidpiston, and to enter said forward chamber through an inlet port duringthe return travel of said piston; inlet vent means for selectivelyadmitting pressurized gas into said rearward chamber for driving saidpiston forward; control means operable when said piston reaches theforward limit of its travel for closing said inlet vent means; saidsliding bearing means normally providing a seal with said piston rod,and said bearing means and piston rod cooperatively providing an outletvent for exhausting driving gas from said rearward chamber when saidpiston closely approaches the forward limit of its travel; and drivemeans operable for returning said piston to its rearward position whenthe driving gas has been exhausted.
 2. A pump as claimed in claim 1wherein said control means is operable in response to a drop in liquidpressure at the forward end of said forward chamber which results fromexhausting said driving gas from said rearward chamber through saidoutlet vent and releasing the liquid from said forward chamber throughsaid outlet part.
 3. A pump as in claim 1 wherein said sliding bearingmeans consists of an elongated cylindrical member extending rearwardfrom the back side of said rear chamber and having a concentric centerbore matching, in close fitting relationship, the piston rod outerdiameter; said elongated member having an exhaust port extendingradially from said center bore through said elongated member wall; saidseal consisting of an O-ring seal acting between said elongatedcylindrical member and said piston rod, and in sliding contact with saidpiston rod; said piston rod having a section of reduced diameter; andsaid outlet vent being provided through the cooperative action of saidpiston rod, bearing means, and seal, said rod reduced secTionsimultaneously engaging said seal and said exhaust port as the pistonclosely approaches the forward limit of its travel.
 4. A pump as inclaim 3 wherein: said O-ring seal and said exhaust port are located atthe piston rod entrance to the rear chamber thereby providing an exhaustpath of minimum restriction.
 5. A positive displacement pump forselectively dispensing a liquid, comprising: a cylinder having areciprocating piston, a piston rod extending rearwardly from saidpiston, and sliding bearing means supporting said piston rod; aconcentric cylinder resilient seal coupled between said piston and thecylinder wall which separates said cylinder into a forward chamber forcontaining a liguid to be dispensed, and a rear chamber; means forallowing said liquid to exit from said forward chamber through an outletpart during the forward travel of said piston and enter said forwardchamber through an inlet part during the return travel of said piston; asource of pressurized gas for driving said piston forward; normallyclosed inlet vent means for admitting the pressurized gas from saidsource into said rear chamber; normally open outlet vent means forexhausting the pressurized gas from said rear chamber; manually actuablecontrol means for selectively opening said inlet vent means andsimultaneously closing said outlet vent means to drive the pistonforward; separate control means operable when said piston reaches theforward limit of its travel for simultaneously closing said inlet ventmeans and opening said outlet vent means; said sliding bearing meansnormally providing a seal with said piston rod, and said bearing meansand piston rod cooperatively providing an outlet vent for exhaustingdriving gas from said rear chamber only when said piston closelyapproaches the forward limit of its travel; and drive means operable forreturning said piston to its rearward position when the driving gas hasbeen exhausted.
 6. A pump as in claim 5 wherein said separate controlmeans is operable in response to a drop in liquid pressure at theforward end of said forward chamber, said drop in liquid pressureresulting from the combined effects of the exhausting of said drivinggas from said rear chamber through said outlet vent and the release ofthe liquid from said forward chamber through said outlet part.
 7. A pumpas in claim 5 with a highly restricted bleeder hole consisting of asmall diameter hole passing through the wall of said rear chamber, forslowly venting the driving gas from said rear chamber and therebyallowing said piston to return to its rearward limit.
 8. A pump as inclaim 1 with an adjustable sleeve for limiting the piston return therebyproviding an adjustable control of the piston displacement withoutaffecting the cooperative action of said piston rod, bearing means, andseal.
 9. A pump as in claim 5 with an adjustable sleeve for limiting thepiston return thereby providing an adjustable control of the pistondisplacement without affecting the cooperative action of said pistonrod, bearing means, and seal.
 10. A pump as in claim 5 wherein the saiddrive means operable for returning said piston consists of a springdriven piston return.
 11. A pump as in claim 5 wherein said drive meansoperable for returning said piston is a spring driven piston returncomprising: an adjustable sleeve for limiting the return travel of saidpiston and thereby providing displacement control, composed of acylindrical housing having a counter-bored interior end wall; acylindrical spring retainer disposed within the end of said housing andadapted to engage the raised circumferential portion of said housing endwall; a spring disposed within said housing and having one end receivedby said spring retainer; a piston rod concentric with said spring, saidspring retainer having a central opening through which said piston rodslidingly passes; and a retainer member secured to tHe end of saidpiston rod for preventing said piston rod from being withdrawn from saidspring retainer; the action being such that when the piston is returned,the piston rod is free to overtravel beyond the normal fully compressedposition of the spring, temporarily extending into the recessed centralportion of said housing end wall, thereby avoiding an undesired reboundof the piston rod from the housing end wall.
 12. A pump as in claim 11wherein the said adjustable sleeve has radial ribs projecting from itsinside surface thereby providing guidance and stability for the saidspring and piston rod.